PCV System: Operation

With engine operating, fresh air enters positive crankcase ventilation system through air cleaner assembly. Fresh air flows through crankcase breather and into rocker arm/valve compartment.

Entering fresh air combines with blow-by gases and unburned air/fuel mixture of crankcase. Combined gases are drawn into carburetor, through PCV valve, by manifold vacuum. Crankcase gases mix with air/fuel mixture and are burned in the combustion chamber. See Fig 1 .

Fig 1: Positive Crankcase Ventilation System (Typical)

PCV valve is held closed by spring pressure when engine is not running. This prevents hydrocarbon fumes from collecting in intake manifold, resulting in hard starting. See Fig 2 .

Fig 2: Cutaway View of a Typical PCV Valve

When engine is running, manifold vacuum pulls PCV valve open allowing crankcase fumes to enter intake manifold. A baffle in rocker arm cover prevents engine oil from being drawn into intake manifold.

If engine backfires through intake manifold, PCV valve closes and prevents any flow of gases through it. This is to prevent ignition of fumes in crankcase.